Kate Middleton has given her first speech. Cue enormous excitement in the media and huge praise. According to one source, she delivered an "assured" performance to "rave reviews". The speech in fact was a few tremulous sentences in which she thanked the charity for inviting her, described its important work, and mentioned missing William.

But while the publicity for the very worthwhile children's hospice can't be faulted, the attention to Kate herself is more problematic, especially in reviving such strong memories of the early press adulation and scrutiny of Princess Diana. On this occasion the media homed in on Kate's dress (borrowed from her mother) and her body (worryingly thin) and drew attention to her natural "gift" with children. It was pure Diana re-run.

Diana had only to turn up at an event to trigger an avalanche of speculation about anorexia or to bend down among the children to be endowed with elements of compassion bordering on the saintly. All this was on display with Kate. Her pencil-thin appearance has led to speculation about whether it is her separation from William or her childlessness that is the cause. As for her compassion, parents who had shaken her hand queued up to vouch for her exceptional talents in this regard. The desire to find a new Diana was palpable.

The media's scrutiny of Diana is thought to have played a part in the disintegration of her marriage; so should we be concerned that the focus on Kate might have similar consequences? In fact, beneath superficialities, the differences between the two women are more striking than the similarities. When Diana set out, she was much younger than Kate and emotionally vulnerable. Her solo appearances came as her marriage was disintegrating and she was desperate to find a role. On occasions she betrayed her turmoil, as with her "everyone needs a hug" speech.

There's no immediate danger of Kate Middleton going off like a loose cannon, because the biggest difference with Diana is just how closely aligned Kate's every movement and utterance is with the "firm", as Diana used to call the royal family. Unlike Diana, this is a woman well-briefed, and carefully supervised.

In this diamond jubilee year the Queen is reaping the enormous benefits of longevity. Just by surviving this long, she is now revered, whatever she does and however difficult she might have been in younger days. She is also benefitting from the amnesia that has settled upon a carefully managed nation, who have forgotten their anger at the treatment of Diana. This amnesia even extends to Camilla, who with the occasional well-planned charity appearance – and a charm offensive on the media establishment (she has recently hosted dinners for the Women of the World festival) – is presented as an acceptable consort for a future King. There have been repeated photo opportunities showing Kate with the Queen or Camilla, or both, like a recent Hello cover showing the three on an outing to Fortnum & Mason. The emphasis is on mutual acceptance and unity.

These photo opportunities tell what's at stake with Kate's public profile. We are used to the monarchy being matriarchal. The symbolism the Queen carries is of "soft" power: she is the head of a family – her own and the nation. She has devoted her life to doing her duty, while keeping her opinions to herself. She is the embodiment of tradition and continuity. These are "female" attributes, and less likely to arouse resistance than a patriarchal embodiment of royal power.

However, the Queen's longevity also arouses anxiety about the future. The next generation of women are pivotal in making monarchy acceptable to Britain in the 21st century. It's the women doing their duty, keeping their heads down, devoted to their husbands and children, who will present the acceptable face of monarchy – not a querulous, tetchy, demanding male.

Kate is too valuable to be left alone as Diana was. The press are giving Kate celebrity-sized attention and with that comes celebrity-sized pressure. But this time, it seems, the firm are ready for it.